Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO90/15188 - 1 - 2 0 5 7 8 7 9 PCT/CA90/00186
PRESSURE CONTROL FORMING SECTION:
This invention is concerned with paper making machines of the
type having a "flat wire" or "open wire" forming section, which
includes means to remove water from the stock by the use of suction
from beneath the forming fabric.
In this type of machine, as opposed to "twin wire" machines, or
"gap formers", an aqueous slurry known as the stock, which contains
both fibers and other substances in an amount of from about 0.1% to
1.5% by weight, is fed from a head box slice onto a single moving
forming fabric. Water is progressively removed from the stock
downwardly through the forming fabric in what is known as the
"forming section" of the paper making machine. In this forming
section, a variety of drainage devices are used, until the stock
contains from about 2% to about 4% by weight of solid material. At
that point, the distribution and orientation of the fibers and other
solids in the still very wet stock is largely determined, and will
not change very much in the remaining paper forming steps unless
other devices such as a dandy roll, or "top wire", is brought into
contact with the stock. Thus at this point the formation of the
paper is largely completed.
In outline, a conventional open wire forming section includes a
forming fabric which is supported at the head box slice end by a
breast roll, which is followed in sequence by a "forming board" and a
series of drainage devices, which may be drainage foils or table
rolls, and suction boxes. More recently, forming sections have
included a forming board followed by suction boxes of the Isoflo
(Trade Mark) type described by Johnson, in U.S. Patent 4,140,573.
These suction boxes heretofore have been distributed along the length
of the forming section with gaps, or undrained spaces, in between them.
The one reported attempt to use vacuum assisted drainage for the
full length of an open wire forming section appears to have been a
failure. Such a paper making machine is described by E.J. Justus in
U.S. 3,052,296 (issued in 1962, assigned to Beloit Iron Works). As
described by Justus, the forming fabric is to be supported on a
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"continuous or substantially uninterrupted" series of suction boxes,
starting as near to the head box slice as is practicable. These
suction boxes are provided with a foraminous surface to support the
forming fabric, for which several designs are proposed. Justus
proffers several advantages for such a machine: an increase in fiber
retention on the forming fabric of up to 70%, as compared to the
usual figure of less than about 50%, reduced wire marking on the
paper, and "better" paper. A further point made by Justus is that
his essentially flat surfaced suction boxes do not cause the
phenomenon known as "kick-up" in the stock associated with the table
rolls then used as the primary dewatering devices. Kick-up results
from the vertical deflection of the forming fabric caused by the
suction produced by the roll as described in U.S. 2,928,465. When
kick-up occurs, what is observed is an essentially vertical movement
of both the forming fabric and the stock carried on it in the
vicinity of a table roll: this movement can become so violent that
it will literally lift the stock off the forming fabric. Such an
occurrence is not conducive to the making of good paper. In a later
communication originating from Beloit Iron Works (reported by P.
Wrist in "The Formation and Structure of Paper", British Paper and
Board Makers Association, London, England, 1962, at pages 863, 864)
it is noted that although many of the benefits proffered by the
all-vacuum assisted drainage technique proposed by Justus indeed are
obtained, nevertheless "the formation of the (paper) sheet
deteriorated to an unacceptable level." (Communication to P. Wrist,
from Beloit Iron Works). In other words it proved to be impossible
to make acceptable quality paper using the modified paper making
machine proposed by Justus. Perhaps as a consequence of this
failure, this approach to stock dewatering was not pursued further.
Even Justus turned his attention to other methods (e.g. as in U.S.
3,102,066).
It has now been realized that the failure of the Justus attempts
may be directly attributed to at least two seemingly unrelated
causes. First, Justus in setting out to avoid the then known
problems of heavy suction and kick-up becoming prevalent with table
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rolls (and which were becoming a handicap serving to limit paper
making speed since as the linear speed of the forming fabric
increased the suction and kick-up effects become more violent)
endeavoured to eliminate all stock agitation in the forming section.
It has now been known for some time that improved paper making
operations can result if some deliberate and controlled agitation is
introduced into the stock on the forming fabric whilst it is still in
a highly fluid state.
It has now been discovered that the precise spacing of the
devices used to generate stock agitation has a very important effect
on paper sheet quality. When the devices are spaced apart in a
uniform manner, they act in a periodic or harmonic relationship to
each other, so that later devices (that is, ones further from the
head box slice) can either reinforce and add to the stock agitation
produced by earlier devices, or diminish and dampen that agitation.
This provides a controlled and uniform stock agitation that is both
easily generated and easily controlled, to benefit the paper sheet
formation.
Second, Justus recommends to use a vacuum level rising to a
figure of 5 cms of mercury at what Justus calls "the 3% point". This
is the point at which the solids content of the stock is
approximately 3%, and broadly corresponds to the end of the forming
section in a more conventional machine. Thus at the end of the
forming section Justus is advocating a vacuum of some 70 cms of water
gauge. It is now known that this is also a mistake, since with
dewatering devices somewhat similar to those advocated by Justus a
far lower level of vacuum is often sufficient, rising to a value of
no more than 50 cms of water at the end of the forming section. The
static drainage device known as an Isoflo, described by Johnson in
U.S. 4,l40,573 mentioned above, is suitable for this purpose.
Finally, Justus recommends vacuum assisted drainage in the first
drainage boxes adjacent the head box slice. A low level of vacuum is
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recommended. This is believed to be a further mistake, in that
vacuum assitance will accelerate the drainage at a point close to the
head box slice.
It has now been discovered that if first, both a desired and a
controlled level of agitation is maintained in the stock throughout
the full length of the forming section, and second, the drainage
velocity through the forming fabric is controlled, so that the rate
of stock drainage can be made more uniform over the whole length of
the forming section, improved paper formation will result.
Furthermore, by carefully controlling the pressure in the drainage
box below the forming fabric, the rate of stock drainage can be
controlled so that if not uniform for the full length of the forming
section, it is at least adjustable and can be optimised to levels
that are more appropriate for the grade of paper being made.
Thus in a first broad aspect this invention provides a method
for improving paper formation on a paper making machine having an
open surface forming section, including at least a travelling
continuous forming fabric which passes over a breast-roll adjacent a
head box having a head box slice through which aqueous stock is
deposited onto the forming fabric, and a plurality of stock
dewatering devices beneath the forming fabric which are provided with
white water drainage means, in which forming section the solids
content of the stock deposited from the head box through the head box
slice onto the forming fabric rises from an initial low value to a
value of from about 2% to about 4%, comprising the steps of:
(i) discharging onto the moving forming fabric an aqueous
stock; and
(ii) causing the forming fabric to move over a forming
section including:
(a) a drainage box comprising a plurality of compartments each
provided with a separately controlled air supply means and
an air pressure tight drainage means;
(b) a foraminous support surface for the open surface forming
fabric which provides an air pressure tight seal between
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the forming fabric and the drainage box compartments, and
which provides a path through which the forming fabric moves which
causes a controlled level of uniformly spaced periodic harmonic
agitation within the stock on the forming fabric; and
(c) controlling the air pressure in the drainage box
compartments to a value that changes along the length of the forming
section from an initial positive value adjacent the head box slice
sufficient to hinder water drainage through the forming fabric but
insufficient to interfere with paper formation on the forming fabric
to a negative value below ambient atmospheric pressure of no more
that 50 cms water gauge at the end of the open surface forming
section.
Preferably the positive pressure adjacent the head box slice is
no more than 2~ cms water gauge above ambient atmospheric pressure.
Preferably, the drainage box comprises either a plurality of
contiguously adjacent drainage boxes along the length of the forming
section, each extending across the width of the forming fabric, or a
single drainage box extending the full length of the forming section
which is provided with a plurality of pressure-and vacuum-tight
transverse divisions between each of which a separate controlled
pressure is applied, and each of which is provided with a separate
pressure tight drainage means.
In a second broad aspect this invention provides in a paper
making machine having an open surface forming section, including at
least a travelling continuous forming fabric which passes over a
breast-roll adjacent a head box having a head box slice through which
aqueous stock is deposited onto the forming fabric, and a plurality
of stock dewatering devices beneath the forming fabric which are
provided with white water drainage means, in which forming section
the solids content of the stock deposited from the head box through
the head box slice onto the forming fabric rises from an initial low
value to a value of from about 2% to about 4%, an apparatus for
improving paper formation consisting essentially of in combination:-
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(a) a drainage box located beneath the forming fabric andextending from a point adjacent the head box slice to the
end of the forming section;
(b) a foraminous support surface for the open surface forming
fabric on the drainage box; and
(c) air supply means, including both vacuum pump means, air
pressure pump means, and air pressure control means,
whereby the air pressure in the drainage box is controlled;
wherein:
(i) the drainage box comprises either a single drainage
box divided into a plurality of separate compartments
by a plurality of air-tight divisions extending across
the width of the drainage box, each compartment of
which is provided with a separate air supply means and
a separate air pressure tight drainage means or a
plurality of separate suction boxes, each of which
extends across the width of the forming fabric, and
each of which is provided with a separate air supply
means and a separate air pressure tight drainage
means;
(ii) the foraminous support surface provides apertures
through which the forming fabric drains and a path
through which the forming fabric moves which will
cause a controlled level of uniformly spaced periodic
harmonic agitation within the stock on the forming
fabric, and provides an air pressure tight seal
between the drainage box compartments and the forming
fabric; and
(iii) the air supply means is controlled to provide an air
pressure in the chambers of the drainage box which
decreases from a positive value above ambient
atmospheric pressure, adjacent the head box slice,
sufficient to hinder water drainage from the stock
through the forming fabric but insufficient to
interfere with paper formation on the forming fabric,
to a negative value of no more than 50 cms water gauge
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below ambient atmospheric pressure at the end of the
forming section.
Preferably the positive pressure is no more than 25 cms water
gauge above ambient atmospheric pressure~
Preferably, the drainage box comprises either a plurality of
contiguously adjacent boxes, each of which is provided with a
separately controlled pressure means, or a single box extending the
full length of the forming section, which is provided with pressure-
and vacuum-tight transverse divisions, the space between each of
which is provided with a separately controlled pressure means,
Preferably in both of these broad aspects of the invention, the
foraminous support surface on the drainage box comprises a plurality
of static support elements having support faces for the forming
fabric including ones chosen to provide a desired level of agitation
in the stock on the forming fabric from the head box slice to the
other end of the forming section. In a preferred embodiment, the
static support elements are so placed as to utilize the harmonic
nature of the agitation which they generate in the stock, thereby
controlling the nature and amount of that agitation. In a more
preferred embodiment, these two effects are combined at least in that
part of the forming section near the head box slice.
The invention will now be described by way of reference to the
attached figures in which:
Figure l shows diagrammatically the initial part comprising the
forming section of a paper making machine;
Figure 2 shows diagrammatically a typical water drainage rate
profile for the forming section shown in Figure l;
Figure 3 shows a forming board and combined foil unit;
Figure 4 shows several support blades;
Figure 5 shows a so-called Isoflo unit;
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Figure 6 shows schematically harmonic stock agitation associated
with a series of foils;
Figure 7 shows diagrammatically the initial part of a paper
making machine modified according to one aspect of this invention.
In these Figures, relevant like parts have been given the same
numbers.
In Figure l, the paper making machine is shown, incorporating a
forming fabric l, which moves in the direction of the arrows shown at
lA and lB. The forming fabric moves over a breast roll 2, and
various tensioning and idling rollers 3. The stock is deposited onto
the forming fabric l from the head box shown diagrammatically at 4,
through a slice 5, which extends across the forming fabric l.
Beneath the forming fabric in the dewatering zone are placed a
sequence of drainage devices 6, 7, 8, 9, 10, 11 and 12, provided with
white water drains 15, l6, l7, l8 and l9. The first of these
drainage devices, 6, comprises a forming board, the second, 7,
comprises an open foil unit, and the remainder are so-called Isoflo
units (Trade Mark). Boxes 8 to 12 are also provided with a
controlled vacuum, through the vacuum pipes 20, 21, 22, 23 and 24
respectively. The vacuum applied will typically range from zero to 5
cms water gauge in box 8, to no more than 50 cms water gauge in box
l2; the white water drains 15, 16, 17, 18 and 19 contain suitable
vacuum legs. A key feature, from the aspect of this invention, is
that not all of the forming section is being actively drained. The
drainage and suction boxes are separated by the spans marked a, b, c,
d, e and f which represent undrained areas, apart from any water
which may happen to drain through under gravity. In the machine
shown, these spans represent nearly 30% of the total area of the
forming zone. At the head box slice S the entire stock is delivered
to the forming fabric. It then passes over the forming board 6, and
foil unit 7, which serve to remove a large amount of water from the
stock. Over the remaining Isoflo units 8 through 12 the balance of
the freely available water is removed.
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The rate at which water is removed from the stock in a forming
section of the type shown in Figure l varies widely with different
paper types. A typical drainage rate is depicted schematically in
Figure 2. In this Figure, the vertical axis represents percentage
water removed, based on the total removed in the forming section
only. It is to be understood that at the end of the forming section
the stock is still very wet, and will contain some 95% or more water.
The horizontal scale corresponds roughly to the dimensions of Figure
l. The various numbered boxes 5 through l2 correspond to the same
units as in Figure l. The interesting feature of this graph, from
the point of view of this invention, is that it indicates very
clearly that the rate of water removal is far from uniform. The
shape of line A indicates that the rate of drainage of the stock
passing over the forming board 6 and foil unit 7 is a higher than for
the remaining Isoflo units. These first two drainage elements handle
some 61% of the total water removed in the forming section, even
though these units occupy only some 25% of the forming section
length. If the rate of water drainage were uniform, the plotted
points would all fall on the dotted line B. This invention seeks to
control the rate of water drainage in order to make it more uniform
and at least approach line B.
In the machine of Figure 1, which is typical of existing prior
art machines, three different forms of drainage element are used, in
sequence away from the head box slice 5. The first of these is a set
of conventional flat forming board blades associated with an open
unrestricted drainage box 6.
The drainage elements 25, associated with the unrestricted
drainage box 7, are conventional foil blades broadly conforming to
the design shown in section in Figure 4A. These foils comprise a
supporting bar 26 with a tee-shaped head, onto which is slid the foil
blade proper, 27. This includes a flat face 28 onto which the
forming fabric l rests, and a divergent trailing face 29. In the
Figure, the divergent angle Q is shown exaggerated for clarity.
Generally it is far smaller than it is shown, ranging from about l
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degree to about 5 degrees, with angles of 2 to 2.5 degrees being
commonly used. As the forming fabric moves over the foil in the
direction of the arrow lA, water is sucked from the stock through the
forming fabric as a consequence of hydraulic phenomena created in the
nip provided by the trailing face 29.
In boxes 8 through 12 a so-called Isoflo unit is used, which is
described in detail in Johnson, U.S. 4,140,573. This is shown in
Figure 5 (which corresponds broadly to Johnson's Figure 4), and can
be seen to incorporate two groups of static devices 30 and 31.
Devices 30 and 31 are each supported on a tee-bar 26; these tee bars
26 are supported across the width of the box by suitably placed
supports 33. Although similar in appearance to the foil blades of
Figure 4A, the static devices 30 and 31 differ in two separate ways.
The top faces of all of these devices which bear against the forming
fabric 1 are generally planar and either in the plane of the forming
fabric (devices 30) or a little below it (devices 31). As shown in
Figure 5 the vertical lowering of the devices 30 is indicated at A,
which is exaggerated for clarity. In practice, this distance
generally will range from about 0.5 mm to about 5.0 mm. The forming
fabric in moving over such a foraminous surface is drawn down by the
vacuum and undulates between successive devices 30. The intervening
devices 31 are so placed vertically as to provide a water seal to the
underside of the forming fabric. Sealing elements, not shown, are
also provided along the sides of the boxes in between the drainage
devices, parallel to the sides of the forming fabric. Water is drawn
from the stock through the forming fabric by the application of
vacuum to the boxes 8 through 12, and leaves the boxes through drains
15 through 19, each of which also provides a suitable vacuum seal.
There is a further feature which is common to both of these
forms of static drainage devices. Figure 6 shows diagrammatically
the harmonic, or periodic, stock agitation that can be generated by a
regular and uniform spacing of the vertical pulses generated by foil
blades supporting a forming fabric. In Figure 6, a small section of
the forming fabric 1 is shown moving in the direction of arrow lA.
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The forming fabric passes over a series of foil blades all uniformly
spaced apart by the distance Y, as indicated between foil blades 34
and 35 mounted on the tee bars 36 and 37. Because the stock
agitation is generated by vertical movement of the forming fabric
caused by the foil blades, which are each spaced apart by the
constant distance Y, the area of vertical stock agitation shown by 61
is followed by another similar area 63. Similarly, the quiescent
zone 62 is followed by another quiescent zone 64, following the area
63. As Figure 6 indicates, both the areas of vertical agitation 61
and 63 and the zones of quiescence 62 and 64 are each spaced apart at
the same distance Y. As shown in Figure 6, with no foil blade on the
tee bar 40, vertical agitation of the stock still occurs at the
location 65, which is differently shaded in Figure 6 to emphasize
that there is no foil blade on tee bar 40, and the amplitude of the
agitation at the location 65 is somewhat less than is obtained with a
foil blade in place on tee bar 40. The occurrence of this activity
in the vicinity of the tee bar 40 (which has no foil blade) is
referred to as occurring at a "ghost blade". It is also important to
note that these areas of agitation and quiescence in the stock do not
move with the forming fabric, but rather remain in essentially the
same place. Normal activity is restored at 67 after a further
quiescent zone 66 by foil 43 mounted on T-bar 44.
For the Johnson Isoflo device shown in Figure 5, the area of the
stock vertical agitation is due to the downward deflection of the
fabric as it moves from fabric support surfaces 30 to surfaces 31,
and periodicity, noted again at Y, similar to that of Figure 6 is
observed.
In Figures 3, 4 and 7 are shown in detail particular embodiments
of the improvements contemplated by this invention.
In the forming section shown in Figure 1 there are two gaps, a
and b, in the forming board and foil unit area. In the revised
forming board of Figure 3 both of these gaps are eliminated. In this
unit, the forming fabric 1 still enters the forming section over the
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breast roll 2 and under the head box slice 5. Immediately adjacent
the slice 5 are four contiguous drainage boxes 46, 47, 48 and 49.
Unlike boxes 6 and 7 these are not open and free draining. Each box
is sealed and provided with a drainage leg 53, 54, 55 and 56, and
also is provided with air inlet pipes 57, 58, 59 and 60 which include
air pressure control means (not shown). The forming board surfaces
used comprise a T-bar as at 30, onto which an elongate blade is
mounted, for example the blade shown at Figure 4B, and which extend
the full width of the forming fabric. Between these blades sealing
elements (not shown) are placed to seal the sides of the forming
fabric to the foraminous surface of the drainage box. Generally the
first blade, 61, which supports the forming fabric in the region
where the stock jet exiting the slice impacts onto the forming fabric
l, is wider than the remaining blades as at 62. It is now known that
this need not be so, and a blade substantially the same width as the
others can be used. In order to maintain pressure sealing, it is
also necessary that blades be present as shown directly above the
dividing walls 50, 51 and 52.
The shape of the top surfaces of these blades and their
placement is of importance. It is now known that almost any surface
used to support a forming fabric, even at this very early stage of
paper formation, has an observable effect on the stock on the forming
fabric. The characteristics of the surface can therefore be chosen
to produce agitation in the stock which can range from sufficient
effectively to lift the stock bodily off the forming fabric and
ranging downwardly through visible macroscopic agitation to
microscopic agitation which can only be observed by using careful
photography and strobe illumination. Furthermore, as is already
discussed above, these blades can be placed to utilize the harmonic
nature of the agitation. Both of these concepts are utilized herein.
The blades are placed to utilize harmonic phenomena, and to induce
the required microagitation, at least as far as the blade 32. A
simple narrow flat blade causes but little agitation. A foil blade,
as in Figure 4A based on the original Wrist ideas, will probably
cause too much agitation for the area adjacent the head box slice.
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Further, since a foil blade drains the stock as a consequence of
hydraulic phenomena in the nip angle Q, the rate at which it drains
the stock is somewhat uncontrollable. The last blades, on box 49,
could perhaps be foils. A third option is shown in Figures 4B and
4C, which is derived from the agitator blade described by Johnson, in
U.S. 3,874,998. In each case, the blade 65 or 66 mounts on a T-bar
by way of the slot 64. Referring first to Figure 4B, the blade 65
has a central depression 67 in its top surface, so that a
cross-machine gap of a flat triangular shape is created below the
plane of the forming fabric. Alternatively, as shown in Figure 4C,
this depression rather than being triangular, can be a shallow
concave shape as at 69. In both of Figures 4B and 4C, as is shown in
Figure 4B, water enters this depression from the stock on the forming
fabric 1, it re-enters the stock, as indicated by the arrows 68 in
Figure 4B. Careful choice of each of the distances Yl~ Y2 and z then
controls the amount of agitation imparted to the stock. The amount
of agitation that is needed at this early stage of paper formation is
small, in the microagitation range mentioned earlier, and therefore
it is possible that not all of the blades will have a depression,
since, as is noted above, a flat-blade surface also causes some
agitation. The first blade immediately following the head box slice
generally is flat surfaced.
Typical dimensions for a blade such as those shown in Figure 4B
or Figure 4C when used to cause microagitation are:
(i) total width (i.e. Yl + Y2 + Z): 25 mm to 75 mm
(ii) flat surface width: generally Yl and Y2 are equal, but are
not necessarily so; the minimum for each is about 5 mm,
with a value of about lO mm being preferred
(iii) the width, z, of the depression; 15 mm to 65 mm
(iv) the depth, x, of the depression: 0.25 mm to 2.5 mm, for
both a triangular and a curved depression.
These dimensions are given as exemplary and do not limit the
scope of this invention, as other dimensions may be found useful in
particular circumstances.
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The preferred value for the depression width z is that it is
about half the total width of the blade. This then leaves adequate
leading and trailing flat portions (Yl and Y2) to get a water seal
onto the blade in these areas. As the depth also affects the amount
of agitation, a wider blade will not necessarily require a deeper
depression. In many cases it is found that if the blade is widened
then the depth, x, should not be changed, although the width z will
generally increase, to maintain it at about half the total blade
width. In selecting a blade for a given circumstance, some care is
needed. The narrowest blade that gives adequate support should be
used. Similarly, the shallowest depression should be used that is
needed to cause the desired amount of agitation. If the blade is
made too wide, and the depression is made too deep, then the level of
agitation can go far beyond the microagitation needed in this area of
the forming fabric to a level where the forming fabric with the stock
on it lifts clear of the blades. Although it is simpler to use a
blade with a single depression, it is realized that in certain
circumstances a wider blade with more than one depression might be
desirable. If such a blade is used, then the central flat portion
between the depressions should be about as wide as the leading and
trailing flat surfaces.
Turning now to Figure 7, it can be seen that this represents the
full length of the forming section of Figure l, but with two main
changes. First, the forming board unit 6 and the foil unit 7 are
replaced by the unit shown in Figure 3. Second, the various Isoflo
drainage boxes 8, 9, 10, 11 and 12 and the boxes 46, 47, 48 and 49
have been incorporated into one full-length drainage box 100. As
shown this is a continuous single box the full length of the forming
section. Such a lengthy unit is cumbersome and would present
engineering problems (but it does permit easy placement of suitable
dividing walls, for example if the paper type being made is changed).
Alternatively, a sequence of contiguous boxes can be used, each
provided with its own pressure and drainage pipes, and pressure
control means. It is also to be noted that there is a forming fabric
support placed directly over each dividing wall in the box lOO. For
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the Isoflo units, 8, 9, 10, 11 and 12 these blades have to be an
upper one, that is a blade 30 in Figure 5.
The overall consequences of this form of construction can now be
considered. In the forming section shown in Figure 7, unlike that in
Figure 1, the machine side of the entire length of the forming
section from the first support blade of the forming board 61 to the
last support element 70 of the Isoflo unit 12 is hydraulically sealed
and there are no free drainage gaps remaining. Further, by using the
various pressure pipes 20 to 24 and 57 to 60 the pressure in each
compartment of the box 100 can be separately controlled to a desired
value, either above or below ambient atmospheric pressure. Finally,
by careful choice of the top surface shape and placement of the
support elements, the path through which the forming fabric moves can
be controlled to induce a desired level of agitation in the stock.
Generally this will increase from an almost invisible level of
micro-agitation near the head box slice 5, to visible induced
macroagitation over much of the remainder.
Due to the hydraulic seal between the forming fabric and the
support surface provided by the support elements and the sealing
members, the pressure in the various compartments of the box lO0 can
also be controlled in such a way that a far more uniform drainage
rate can be obtained. If arbitrarily "zero" is taken as ambient
atmospheric pressure, then a typical pressure profile would range
from a positive figure of up to no more than 25 cms water gauge in
box 46, to a negative figure of down to no more than 50 cms water
gauge at box l2. Such a pressure profile would pass through zero, or
ambient pressure, at about box 49 or box 8. By utilizing this
positive applied pressure in the early boxes, the high rate of
drainage normally associated with this part of the forming section
can be significantly reduced, so that the overall drainage rate
profile can approach the ideal of the line B in Figure 2. Under
these conditions of controlled agitation and controlled drainage rate
better paper formation is obtained. Further, it appears that stock
retention in the paper also improves.
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Retention is fundamental in paper making. The commonly used
definition in paper making for first pass retention (FPR) is
Head Box Consistency - White Water Consistency x 100
Head Box Consistency
Values for FPR can range from 30% in the case of papers with a
high filler content to over 90% for some long fibered grades.
Several factors affect the FPR including the type of stock, the kind
of forming fabric, the use of chemical retention aids, the amount of
stock agitation, the amount of suction used in forming the paper, and
particularly the velocity induced in the stock by that suction while
forming. Improving retention from q5% to 70% reduces the consistency
of the recirculating white water considerably if the amount of slice
opening is left unchanged. (By "consistency" in this context is
meant the total suspended solids content in percent by weight in the
stock or in the white water, as appropriate). This has beneficial
effects on the entire paper mill and reduces the amount of fiber and
filler loss. Alternatively, the paper maker may cut down on the
slice opening and use less water for forming the paper. Thus one
benefit of this invention, which allows using controlled drainage
rates while still achieving good formation, is to reduce the velocity
of drainage thereby improving retention and wire mark. An
improvement in FPR of up to 20% can be obtained with a forming
section according to this invention.
